The theory of mutations is a starting-point for direct investigation, while the general belief in slow changes has held back science from such investigations during half a century. Coming now to the subdivisions and headings under which my material is to be presented, I propose describing first the real nature of the elementary species and retrograde varieties, both in normal form and in hybridizations. A discussion of other types of varieties, including [31] monstrosities will complete the general plan. The second subdivision will deal with the origin of species and varieties as taught by experiment and observation, treating separately the sudden variations which to my mind do produce new forms, and subsequently the fluctuations which I hold to be not adequate to this purpose. [32] B. ELEMENTARY SPECIES LECTURE II ELEMENTARY SPECIES IN NATURE What are species? Species are considered as the true units of nature by the vast majority of biologists. They have gained this high rank in our estimation principally through the influence of Linnaeus. They have supplanted the genera which were the accepted units before Linnaeus. They are now to be replaced in their turn, by smaller types, for reasons which do not rest upon comparative studies but upon direct experimental evidence. Biological studies and practical interests alike make new demands upon systematic botany. Species are not only the subject-material of herbaria and collections, but they are living entities, and their life-history and life-conditions command a gradually increasing interest. One phase of the question is to determine the easiest manner to deal with the collected forms of a country, and another feature is the problem [33] as to what groups are real units and will remain constant and unchanged through all the years of our observations. Before Linnaeus, the genera were the real units of the system. De Candolle pointed out that the old common names of plants, such as roses and clover, poplars and oaks, nearly all refer to genera. The type of the clovers is rich in color, and the shape of the flower-heads and the single flowers escape ordinary observation; but notwithstanding this, clovers are easily recognized, even if new types come to hand. White and red clovers and many other species are distinguished simply by adjectives, the generic name remaining the same for all. Tournefort, who lived in the second half of the 17th century (1656-1708), is generally considered as the author of genera in systematic botany. He adopted, what was at that time the general conception and applied it throughout the vegetable kingdom. He grouped the new and the rare and the previously overlooked forms in the same manner in which the more conspicuous plants were already arranged by universal consent. Species were distinguished by minor marks and often indicated by short descriptions, but they were considered of secondary importance.